A huge, uncomfortable and heavy (never actually weighed it, but I estimate it's weight by
at least 7 pounds) blue book (usually comes in a blueish-grey wrapping), published by the
Oxford University Press.
Contains all the words in the ancient Greek language that modern research knows the meaning of, including conjugation and declension info (particularly to irregular nouns, verbs and adjectives, but usually also for regular ones), and some etymological info, as well as many references to proper names of persons, places and mythological figuers.
It's considered the best dictionary of Ancient Greek in the English language, possibly the best in any language (though it has stiff competition with two German ones).
As the name suggests, the Lexicon was composed by two reknowned scholars named Liddell and Scott. Though it had many reprints since then, the 7th and latest edition of the Lexicon was published in 1883.
Later editions of the dictionary are actually 'Liddle-Scott-Jones-McKenzie Greek-English Lexicon' and are for the most part less good than the original.